Studies at our Center indicate that dietary sodium reduces the systemic availability of drugs metabolized by one particular enzyme, CYP3A4. CYP 3A4 is expressed in both liver and intestine, and our studies appear to show preferential increase in CYP 3A4 activity in the intestine during salt loading in normal human volunteers. The aim of this study is to determine, using stable-label methods, the extent of systemic availability of another CYP 3A4 substrate, verapamil, during sodium-defined diets in normal human volunteers.